Liquid crystal display device

ABSTRACT

An alignment film is given a 2-layer structure comprising a photoalignment film that is photoalignable and a low-resistivity alignment film whose resistivity is smaller than that of the photoalignment film. The photoalignment film is formed by a polyimide whose precursor is polyamide acid alkyl ester, the number molecular weight of the photoalignment film is large, and the stability of alignment of the photoalignment film by photoalignment is excellent. The low-resistivity alignment film is formed by a polyimide whose precursor is polyamide acid, the number molecular weight of the low-resistivity alignment film is small, and the resistivity of the low-resistivity alignment film is small. The 2-layer structure alignment film can be maintaining an excellent photoalignment characteristic, so DC afterimages can be controlled.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/943,151, filed on Jul. 16, 2013, which is a continuation of U.Sapplication Ser. No. 12/560,770, filed on Sep. 16, 2009, now U.S. Pat.No. 8,497,002, the contents of which are incorporated herein byreference.

The present application claims priority from Japanese application JP2008-235900 filed on Sep. 16, 2008, the content of which is herebyincorporated by reference into this application.

BACKGROUND

Technical Field

The present invention pertains to a liquid crystal display device andparticularly relates to a liquid crystal display device that is equippedwith a liquid crystal display panel where alignment control ability isimparted to an alignment film by irradiation with light.

Related Art

Liquid crystal display devices have a TFT substrate on which pixelelectrodes and thin film transistors (TFTs) are formed in a matrix, anopposing substrate that opposes the TFT substrate and on which colorfilters are formed in places corresponding to the pixel electrodes ofthe TFT substrate, and liquid crystal that is held between the TFTsubstrate and the opposing substrate. Additionally, liquid crystaldisplay devices form an image by controlling, per pixel, thetransmittance of light by the liquid crystal molecules.

Liquid crystal display devices are flat and lightweight, so they areused for various purposes in many different fields, from large displaydevices in televisions and the like to mobile telephones and digitalstill cameras (DSC). On the other hand, viewing angle characteristics ofliquid crystal display devices are a problem. Viewing anglecharacteristics are a phenomenon where brightness varies or chromaticityvaries between when the screen is seen from the front and when thescreen is seen from a diagonal direction. In-Plane Switching (IPS),where the liquid crystal molecules are moved by a horizontal directionelectric field, has excellent viewing angle characteristics.

As a method of aligning an alignment film that is used in liquid crystaldisplay devices, that is, imparting alignment control ability, therelated art includes a method of aligning the alignment film by rubbing.This is a method where alignment is performed by rubbing the alignmentfilm with a cloth. On the other hand, there is a technique calledphotoalignment where alignment control ability is imparted to thealignment film without contact. IPS does not require a pretilt angle, sophotoalignment can be applied thereto.

In JP-A-2004-206091, there is disclosed photodegradative photoalignmentby irradiation with light represented by ultraviolet light. According tothis, photodegradative photoalignment (1) reduces alignment disordercaused by the complex step structure of the pixel portions and (2)resolves display defects caused by alignment disorder resulting fromdust or disorder of the bristle tips of the rubbing cloth and breakageof the thin film transistors resulting from static electricity duringalignment by rubbing and resolves the cumbersomeness of the process offrequent replacement of the rubbing cloth that is needed in order toobtain homogenous alignment control ability.

However, it is known that, in regard to the point of imparting alignmentcontrol ability to the alignment film, alignment stability is usuallylower in photoalignment in comparison to alignment stability in rubbing.When alignment stability is low, the initial alignment directionfluctuates, which leads to display defects. Particularly in liquidcrystal display devices that use an in-plane switching liquid crystaldisplay panel where high alignment stability is demanded, it is easy fordisplay defects symbolized by afterimages to occur as a result ofalignment stability being low.

In photoalignment, there is, in the LCD process, no step of stretchingthe main chain of a polymer into a straight line as there is in rubbing.For that reason, in photoalignment, the main chain of a syntheticpolymer alignment film represented by a polyimide that has beenirradiated with polarized light is broken in a direction parallel to thepolarization direction, whereby uniaxiality is imparted. The liquidcrystal molecules align along the direction of the long main chain thatremains extending on a straight line without being broken, but when thelength of this main chain becomes short, uniaxiality drops, interactionwith the liquid crystal becomes weak and alignment deteriorates, so itbecomes easy for afterimages to occur.

Consequently, in order to improve the uniaxiality of the alignment filmand improve alignment stability, it is necessary to increase themolecular weight of the alignment film. As a technique for solving this,a photoalignment film material obtained by imidizing polyamide acidalkyl ester can be used. According to this, in the polyamide acid alkylester material, the molecular weight can be kept large even afterimidization without being accompanied by a decomposition reaction to ananhydride and a diamine at the time of the imidization reaction that hadoccurred in the conventional polyamide acid material, and alignmentstability on a par with rubbing can be obtained.

Further, the polyamide acid alkyl ester material does not includecarboxylic acid in its chemical structure, so its LCD voltage holdingratio rises in comparison to the polyamide acid material, and animprovement in long-term reliability can also be ensured.

In order to obtain alignment stability and long-term reliability of aphotoalignment film, application of the polyamide acid alkyl estermaterial is effective, but this material usually has high alignment filmspecific resistance in comparison to the polyamide acid material. Forthat reason, when a direct-current voltage superposes on the signalwaveform that drives the liquid crystal molecules and becomes residualDC, the time constant until the residual DC eases is large and itbecomes easy for this to lead to image persistence (DC afterimages).

SUMMARY

It is an object of the invention to provide a liquid crystal displaydevice that uses a liquid crystal display panel that can reduce thedisappearance time of DC afterimages without impairing the alignmentstability and the long-term reliability of a polyamide acid alkyl estermaterial and can perform high-definition display.

The invention overcomes the above-described problems, and its specificmeans are as follows. That is, a liquid crystal display device of theinvention is equipped with a liquid crystal display panel comprising: aTFT substrate that has a TFT alignment film on the top of its mainsurface on which active elements for pixel selection are formed; anopposing substrate that has a color filter alignment film on the top ofits main surface on which color filters are formed; and liquid crystalthat is sealed between the TFT alignment film of the TFT substrate andthe color filter alignment film of the opposing substrate. Further, theTFT alignment film and the color filter alignment film have a liquidcrystal alignment control function imparted by irradiation with light.

The alignment film material for imparting, with respect to at least oneof the alignment film of the TFT substrate and the opposing substratealignment film on which the color filters are formed, the liquid crystalalignment control function by irradiation with light is configured by a2-component system material comprising a material that is aligned bylight represented by ultraviolet light and a material whose specificresistance is low. After alignment film formation, each of the alignmentfilms is configured by a 2-layer structure comprising a photoalignmentfilm for aligning and a low-resistivity film that does not contribute toalignment.

The material that is aligned by light can comprise a material obtainedby polyimidizing photodegradable polyamide acid alkyl ester. Further, amaterial where this chemical imidization is equal to or greater than 70%can be used.

The low-resistivity material can comprise a material obtained bypolyimidizing a polyamide acid material, but it is not necessary forthis to be photodegradable. Further, a material where this chemicalimidization is equal to or greater than 40% can be used.

According to the invention, the material comprising the 2-componentsystem is phase-separated to give each of the alignment films a 2-layerstructure, the photoalignment component whose alignment stability ishigh is disposed on the liquid crystal layer side and thelow-resistivity component where alignment stability is unnecessary isdisposed on the substrate side, whereby it becomes possible tosimultaneously satisfy alignment stability and a reduction in the timeconstant of DC afterimages resulting from the resistance of thealignment film becoming lower; as a result, afterimage characteristicsof the photoalignment film are significantly improved.

Further, the low-resistivity component disposed on the substrate sidedoes not contribute to alignment, so its molecular weight can be loweredto the utmost extent. For that reason, the margin expands in regard toadjusting the viscosity and concentration of the alignment film varnishwhere the polyamide acid alkyl ester and the polyamide acid have beendissolved in an organic solvent, and, in regard to the method of formingthe alignment film, not only in conventional flexo printing but also ininkjet coating, where it had been difficult to make the alignment filmthick because it has been necessary to make the viscosity of the varnishlow, the margin with respect to making the alignment film thickimproves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an IPS liquid crystal displaydevice;

FIG. 2 is a plan diagram of a pixel electrode of FIG. 1;

FIGS. 3A and 3B are diagrams showing the configuration of an alignmentfilm according to the invention;

FIGS. 4A and 4B are diagrams showing the formation process of aphotoalignment film;

FIGS. 5A and 5B are cross-sectional diagrams of the alignment film ofthe invention;

FIG. 6 shows a DC afterimage evaluation pattern; and

FIG. 7 shows DC afterimage evaluation results.

DETAILED DESCRIPTION

The content of the invention will be described in detail by thefollowing embodiments.

Embodiment 1

FIG. 1 is a cross-sectional diagram showing a structure in a displayregion of an IPS liquid crystal display device. Various structures arebeing proposed and put to practical use for the electrode structure ofIPS liquid crystal display devices. The structure in FIG. 1 is astructure that is currently being widely used; simply put, a pectinatepixel electrode 110 is formed on an opposing electrode 108 formed in aflat solid, with an insulating film sandwiched between the pixelelectrode 110 and the opposing electrode 108. Additionally, the liquidcrystal display device forms an image by controlling the transmittanceof light through a liquid crystal layer 300 per pixel by causing liquidcrystal molecules 301 to be rotated by a voltage between the pixelelectrodes 110 and the opposing electrodes 108. The structure in FIG. 1will be described in detail below. It will be noted that, although theinvention will be described taking the structure in FIG. 1 as anexample, the invention can also be applied to IPS type liquid crystaldisplay devices other than the one shown in FIG. 1, such as a devicewhere the opposing electrode is positioned on an upper insulating filmand the pixel electrode is positioned under the upper insulting film ora device where the opposing electrode and the pixel electrode are on thesame plane.

In FIG. 1, a gate electrode 101 is formed on a TFT substrate 100 formedby glass. The gate electrode 101 is formed in the same layer as a scanline. The gate electrode 101 comprises a MoCr alloy laminated on an AlNdalloy.

A gate insulating film 102 is formed by SiN so as to cover the gateelectrode 101. A semiconductor layer 103 is formed on the gateinsulating film 102 by an a-Si film in a position opposing the gateelectrode 101. The a-Si film is formed by plasma CVD. The a-Si filmforms a TFT channel portion, and a source electrode 104 and a drainelectrode 105 are formed on the a-Si film, with the channel portionbetween the source electrode 104 and the drain electrode 105. It will benoted that an unillustrated n+Si layer is formed between the a-Si filmand the source electrode 104 or the drain electrode 105. This is toestablish an ohmic contact between the semiconductor layer 103 and thesource electrode 104 or the drain electrode 105.

A picture signal line doubles as the source electrode 104, and the drainelectrode 105 is connected to the pixel electrode 110. Both the sourceelectrode 104 and the drain electrode 105 are formed in the same layerat the same time. In the present embodiment, the source electrode 104 orthe drain electrode 105 is formed by a MoCr alloy. When one wishes tolower the electric resistance of the source electrode 104 or the drainelectrode 105, an electrode structure where, for example, an AlNd alloyis sandwiched by a MoCr alloy is used.

An inorganic passivation layer 106 is formed by SiN so as to cover theTFT. The inorganic passivation layer 106 protects particularly thechannel portion of the TFT from an impurity 401. An organic passivationlayer 107 is formed on the inorganic passivation layer 106. The organicpassivation layer 107 protects the TFT and, at the same time, also hasthe function of flattening its surface, so the organic passivation layer107 is thickly formed. Its thickness is 1 μm to 4 μm.

A photosensitive acrylic resin, silicon resin or polyimide resin is usedfor the organic passivation layer 107. It is necessary to form a throughhole 111 in the portion of the organic passivation layer 107 where thedrain electrode 105 connects to the pixel electrode 110, but because theorganic passivation layer 107 is photosensitive, the through hole 111can be formed without using a photoresist by exposing and developing theorganic passivation layer 107 itself.

The opposing electrode 108 is formed on the organic passivation layer107. The opposing electrode 108 is formed by sputtering indium tin oxide(ITO), which is a transparent conductive film, on the entire displayregion. That is, the opposing electrode 108 is formed planarly. Afterthe opposing electrode 108 is formed by sputtering on the entiresurface, the opposing electrode 108 is removed by etching only at thethrough hole 111 portion for allowing the pixel electrode 110 and thedrain electrode 105 to be conductive.

An upper insulating film 109 is formed by SiN so as to cover theopposing electrode 108. After an upper electrode is formed, the throughhole 111 is formed by etching. This upper insulating film 109 is used asa resist to etch the inorganic passivation layer 106 and form thethrough hole 111. Thereafter, ITO, which becomes the pixel electrode110, is formed by sputtering so as to cover the upper insulating film109 and the through hole 111. The sputtered ITO is patterned to form thepixel electrode 110. The ITO that becomes the pixel electrode 110 isalso deposited in the through hole 111. In the through hole 111, thedrain electrode 105 extending from the TFT and the pixel electrode 110are conductive, and a picture signal is supplied to the pixel electrode110.

One example of the pixel electrode 110 is shown in FIG. 2. The pixelelectrode 110 is a pectinate electrode whose both ends are closed. Slits112 are formed between the comb teeth. The planar opposing electrode108, which is not illustrated here, is formed under the pixel electrode110. When the picture signal is applied to the pixel electrode 110, theliquid crystal molecules 301 are rotated by electric force lines arisingbetween the pixel electrode 110 and the opposing electrode 108 throughthe slits 112. Thus, light passing through the liquid crystal layer 300is controlled to form an image.

FIG. 1 is a diagram where this aspect is described as a cross-sectionaldiagram. The slit 112 shown in FIG. 1 is between a pectinate electrodeand a pectinate electrode. A constant voltage is applied to the opposingelectrode 108, and a voltage resulting from the picture signal isapplied to the pixel electrode 110. When a voltage is applied to thepixel electrode 110, as shown in FIG. 1, electric force lines arise andcause the liquid crystal molecules 301 to rotate in the direction of theelectric force lines and control the transmittance of light from abacklight. An image is formed because transmittance from the backlightis controlled per pixel.

In the example in FIG. 1, the planarly formed opposing electrode 108 isdisposed on the organic passivation layer 107 and the pectinateelectrode 110 is disposed on the upper insulating film 109. However,conversely from this, there are also instances where a planarly formedpixel electrode 110 is disposed on the organic passivation layer 107 anda pectinate opposing electrode 108 is disposed on the upper insulatinglayer 109.

An alignment film 113 for aligning the liquid crystal molecules 301 isformed on the pixel electrode 110. In the invention, the alignment film113 has a 2-layer structure comprising a photoalignment film 1131 thatcontacts the liquid crystal layer 300 and a low-resistivity alignmentfilm 1132 that is formed on the bottom of the photoalignment film 1131.The structure of the alignment film 113 will be described in detaillater.

In FIG. 1, an opposing substrate 200 is disposed with the liquid crystallayer 300 sandwiched between the opposing substrate 200 and the TFTsubstrate 100. Color filters 201 are formed on the inner side of theopposing substrate 200. As for the color filters 201, color filters 201of red, green and blue are formed per pixel and a color image is formed.A black matrix 202 is formed between the color filters 201 to improveimage contrast. It will be noted that the black matrix 202 also has arole as a TFT light blocking film and prevents a photocurrent fromflowing in the TFT.

An overcoat film 203 is formed so as to cover the color filters 201 andthe black matrix 202. The surface of the color filters 201 and the blackmatrix 202 is uneven, so the surface is made flat by the overcoat film203.

An alignment film (orientation film) 113 for determining initialalignment of the liquid crystal is formed on the overcoat film 203. Thealignment film 113 of the opposing substrate 200 also, similar to thealignment film 113 of the TFT substrate 100, has a 2-layer structurecomprising a photoalignment film 1131 that contacts the liquid crystallayer 300 and a low-resistivity alignment film 1132 that is formed onthe bottom of the photoalignment film 1131. It will be noted that,because FIG. 2 is IPS, the opposing electrode 108 is formed on the TFTsubstrate 100 side and is not formed on the opposing substrate 200 side.

As shown in FIG. 1, in IPS, a conductive film is not formed on the innerside of the opposing substrate 200. Therefore, the potential of theopposing substrate 200 becomes unstable. Further, electromagnetic noisefrom outside penetrates the liquid crystal layer 300 and affects theimage. In order to eliminate this problem, a surface conductive film 210is formed on the outer side of the opposing substrate 200. The surfaceconductive film 210 is formed by sputtering ITO, which is a transparentconductive film.

FIGS. 3A and 3B are schematic diagrams showing the alignment film 113according to the invention. FIG. 3A is a plan transparent diagram of thealignment film 113, and FIG. 3B is a cross-sectional perspectivediagram. The alignment film 113 of the invention has a 2-layerstructure, with the top side that contacts the liquid crystal layer 300being configured by the photoalignment film 1131 and the bottom sidebeing configured by the low-resistivity alignment film 1132. Thephotoalignment film 1131 is formed by a photodegradable polymer 10 whosemolecular weight is large, and the low-resistivity alignment film 1132is formed by a low-resistivity polymer 11 whose molecular weight issmall.

Because FIG. 3A is a transparent diagram, the photodegradable polymer 10and the low-resistivity polymer 11 can be seen. In actuality, thephotodegradable polymer 10 is present higher than the low-resistivitypolymer 11. In FIG. 3B, the alignment film 113 is formed on the pixelelectrode 110 or the organic passivation layer 107 of FIG. 1. In FIG.3B, the alignment film 113 is shown as being formed on the pixelelectrode 110. A thickness t1 of the photoalignment film 1131 on the topside is about 50 nm, and a thickness t2 of the low-resistivity alignmentfilm 1132 on the bottom side is about 50 nm. The boundary between thephotoalignment film 1131 and the low-resistivity alignment film 1132 isindicated by a dotted line because it is not clear.

FIGS. 4A and 4B are schematic diagrams showing the process by which theliquid crystal is aligned by the photoalignment film 1131. In FIGS. 4Aand 4B, the low-resistivity alignment film 1132 is omitted. FIG. 4Ashows a state where the photoalignment film 1131 has been applied. Thephotoalignment film 1131 is formed by the photodegradable polymer 10.

The photoalignment film 1131 shown in FIG. 4A is irradiated withpolarized ultraviolet light at, for example, an energy of 6 J/cm².Therefore, the photodegradable polymer 10 in the polarization directionof the polarized ultraviolet light in the photoalignment film 1131 is,as shown in FIG. 4B, broken down by the ultraviolet light. That is,broken portions 15 resulting from the ultraviolet light are formed alongthe polarization direction of the ultraviolet light. Therefore, theliquid crystal molecules 301 become aligned in the direction of arrow Ain FIG. 4B.

As shown in FIGS. 4A and 4B, when the main chain of the photodegradablepolymer 10 is short, uniaxiality drops, interaction with the liquidcrystal becomes weak and alignment deteriorates. Consequently, in FIG.4B, even after photoalignment, it is desirable for the photodegradablepolymer 10 to extend as long as possible in the direction of arrow A. Inother words, in order to improve the uniaxiality of the alignment film113 and improve alignment stability, it is necessary to increase themolecular weight of the alignment film 113.

The molecular weight of the alignment film 113 can be evaluated bynumber molecular weight. Number molecular weight is, when polymers ofvarious molecular weights are present in the alignment film 113, theaverage molecular weight thereof. In the photoalignment film 1131, it isnecessary for the number molecular weight to be equal to or greater than5000 in order to obtain sufficient alignment stability.

In order to obtain the photoalignment film 1131 with a large numbermolecular weight, a film obtained by imidizing polyamide acid alkylester can be used. The structure of polyamide acid alkyl ester is asshown in chemical formula (1).

In chemical formula (1), R1 respectively independently represents analkyl group with a carbon number of 1 to 8, R2 respectivelyindependently represents a hydrogen atom, a fluorine atom, a chlorineatom, a bromine atom, a phenyl group, an alkyl group with a carbonnumber of 1 to 6, an alkoxy group with a carbon number of 1 to 6, avinyl group (—(CH2)m-CH═CH2, m=0, 1, 2) or an acetyl group(—(CH2)m-C≡CH, m=0, 1, 2), and Ar represents an aromatic compound.

The characteristic of the polyamide acid alkyl ester is R1 in chemicalformula (1). In the polyamide acid alkyl ester, R1 is CnH2n-1, and n isequal to or greater than 1. When the polyamide acid alkyl ester is usedas the precursor of the photoalignment film 1131, the molecular weightcan be kept large even after imidization without being accompanied by adecomposition reaction to an anhydride and a diamine at the time of theimidization reaction such as had occurred in the conventional polyamideacid material, and alignment stability on a par with rubbing can beobtained.

However, the specific resistance of the photoalignment film 1131obtained by imidizing the polyamide acid alkyl ester is extremely high,such as, for example, about 10¹⁵ Ωcm. When the specific resistance ofthe alignment film 113 is high in this manner, the electric charge withwhich the liquid crystal layer 300 has been charged cannot be let out,and this electric charge causes DC afterimages.

In liquid crystal display devices, alternating-current driving isperformed so as to not charge the liquid crystal layer with an electriccharge, but when the liquid crystal display device displays a particularimage for a particular amount of time, a DC component remains in one ofthe substrates for a certain amount of time, the liquid crystal layer ischarged with an electric charge, and sometimes an afterimage arisesbecause of the electric charge with which the liquid crystal layer hasbeen charged. If the resistance of the alignment film 113 is notextremely large, the electric charge in the liquid crystal can be letout to the pixel electrode 110 or the like through the alignment film113.

However, the specific resistance of the photoalignment film 1131 isextremely large, and the photoalignment film 1131 cannot let out theelectric charge in the liquid crystal in a short amount of time. Theinvention solves this problem by giving the alignment film 113 a 2-layerstructure comprising the photoalignment film 1131 and thelow-resistivity alignment film 1132. That is, the photoalignment film1131 that contacts the liquid crystal layer uses the photodegradablepolymer 10 whose molecular weight is large in order to obtain sufficientalignment stability.

The bottom of the alignment film 113 is configured by thelow-resistivity alignment film 1132 whose molecular weight is small andwhose specific resistance is also small. The resistivity of thelow-resistivity alignment film 1132 is, for example, about 10¹² to 10¹⁴Ωcm. The low-resistivity alignment film 1132 can be formed by a filmobtained by imidizing polyamide acid.

The polyamide acid is expressed by the structure shown in chemicalformula (2).

In chemical formula (2), R2 respectively independently represents ahydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, aphenyl group, an alkyl group with a carbon number of 1 to 6, an alkoxygroup with a carbon number of 1 to 6, a vinyl group (—(CH2)m-CH═CH2,m=0, 1, 2) or an acetyl group (—(CH2)m-C≡CH, m=0, 1, 2), and Arrepresents an aromatic compound.

The polyamide acid differs from the polyamide acid alkyl ester in that,in chemical formula (1) representing the polyamide acid alkyl ester, R1is replaced with H. The polyamide acid undergoes a decompositionreaction to a diamine and an anhydride at the time of the imidizationreaction that forms the polyimide, so the polyimide cannot be given asufficiently large molecular weight. Consequently, a sufficientcharacteristic cannot be obtained for the photoalignment film 1131. Thespecific resistance of the polyimide formed by the polyamide acid is notthat large.

In this manner, by giving the alignment film 113 a 2-layer structure,alignment stability resulting from photoalignment can be ensured, thespecific resistance of the alignment film 113 overall can be controlledto an appropriate value, and DC afterimages can be alleviated.

In order to form the 2-layer structure alignment film 113, the alignmentfilm 113 formation process can be performed without being increased.That is, as shown in FIG. 5A, when a material in which thephotodegradable polymer 10 and the low-resistivity polymer 11 are mixedtogether is applied to a substrate, phase separation occurs such thatthe material that easily adapts to the substrate is formed on the bottomand the other material is formed on top. It will be noted that thesubstrate in FIG. 5A is represented by the pixel electrode 110.

In the present embodiment, that which corresponds to the substrate isthe ITO that forms the pixel electrode 110 or the organic passivationlayer 107. In FIGS. 5A and 5B, the pixel electrode 110 is shown as thesubstrate. The polyamide acid more easily adapts to the ITO that formsthe pixel electrode 110 or the organic passivation layer 107 incomparison to the polyamide acid alkyl ester, so the polyamide acidalways becomes the bottom layer.

Heat of about 200° C. is applied to the resin film formed in this mannerto polyimidize the resin film. Polyimidization is performed at the sametime with respect to both the polyamide acid on the bottom and thepolyamide acid alkyl ester on the top. Consequently, formation of a2-layer alignment film 113 can be performed by the same process as theformation of a 1-layer alignment film 113.

The photoalignment film 1131 on the top stabilizes the alignmentcharacteristic, so it is necessary to raise its imidization ratiobecause it is necessary to increase the molecular weight of thephotodegradable polymer 10. The imidization ratio of the photodegradablefilm 1131 is equal to or greater than 70% and more preferably equal toor greater than 80%. As for the rest of this, the polyamide acid alkylester that serves as the precursor becomes present in the photoalignmentfilm 1131.

The low-resistivity alignment film 1132 on the bottom has norelationship with the photoalignment characteristic of the liquidcrystal, so its imidization ratio may be low. For example, it sufficesfor the imidization ratio to be equal to or greater than 40%. That is,it suffices for the condition of imidization to be set focusing on theimidization of the polyamide acid alkyl ester on the top.

The boundary between the top layer and the bottom layer of the alignmentfilm 113 is not clear. In FIG. 5B, this boundary is indicated by adotted line. In FIG. 5B, the molecular weight of the photoalignment film1131 on the top is larger than that of the low-resistivity alignmentfilm 1132 on the bottom. When the molecular weights of thephotoalignment film 1131 and the low-resistivity alignment film 1132 arecompared, it suffices to compare the molecular weight at the surface ofthe photoalignment film 1131 and the molecular weight at the interfacebetween the low-resistivity alignment film 1132 and the substrate.

DC afterimage characteristics in a case where the alignment film 113with the 2-layer structure comprising the photoalignment film 1131 andthe low-resistivity alignment film 1132 is used as the alignment filmand a case where an alignment film having only the photoalignment filmis used are evaluated. Afterimages are evaluated as follows. That is, ablack-and-white 8×8 checker flag pattern such as shown in FIG. 6 isdisplayed for 12 hours and thereafter returned to a gray color solidhalftone. The halftone gradation is 64/256.

FIG. 7 shows DC afterimage evaluation results. In FIG. 7, the horizontalaxis represents the amount of time after the pattern has been returnedto the gray color solid halftone. The vertical axis represents the levelof the afterimage. On the vertical axis, RR is a state where the checkerflag pattern can be seen well when it has been returned to the halftone,and is no good. R is a state where the checker flag pattern is faint butcan still be seen when it has been returned to the halftone. In FIG. 7,curve A is a DC afterimage characteristic when the alignment filmaccording to the invention is used. Further, curve B is an example of aDC afterimage characteristic when just a 1-layer photoalignment film isused as the alignment film.

It can be said that, when the pattern has been returned to the halftone,there is no problem in terms of practical use even when the level of theafterimage is R if this disappears in a short amount of time. In thecase of the 1-layer photoalignment film, level R when the pattern hasbeen returned to the halftone continues for a long period of time, so apractical problem remains. On the other hand, in the alignment film 113with the 2-layer structure according to the invention, the DC afterimagerapidly decreases, and after the pattern has been returned to thehalftone, the DC afterimage completely disappears in about 17 minutes.

In this manner, the large difference between the case of the 1-layerphotoalignment film and the case of the alignment film of the inventionis that, whereas the DC afterimage continues for a long time in the caseof the 1-layer photoalignment film, the DC afterimage rapidly decreaseswhen the alignment film of the invention is used. In FIG. 7, when thelevels of the DC afterimage 10 minutes after the pattern has beenreturned to the halftone that becomes a rough indication of the DCafterimage, the DC afterimage is 90% in the case where the alignmentfilm is configured by just the 1-layer photoalignment film, but the DCafterimage in the invention becomes equal to or less than 25%, and itwill be understood that the effect of the invention is extremely large.

Incidentally, in order to stabilize the alignment characteristic in thephotoalignment film 1131, it is better for the molecular weight of thephotodegradable polymer 10 to be large. However, when the molecularweight is large, the viscosity of the alignment film varnish becomeshigh. When the viscosity becomes high, flexo printing and inkjet coatingbecome difficult, so the concentration of the material is lowered tolower the viscosity. Therefore, the coating film ends up becoming thin.Consequently, the coating condition for forming the alignment film 113by just the 1-layer of the photoalignment film ends up becoming limited.

In contrast, the invention has the following advantages. That is, thelow-resistance component disposed on the substrate side does notcontribute to alignment, so the molecular weight can be lowered to theutmost limit. For that reason, the margin expands in regard to adjustingthe concentration and viscosity of the alignment film varnish where thepolyamide acid alkyl ester and the polyamide acid have been dissolved inan organic solvent, and, in regard to the method of forming thealignment film 113, not only conventional flexo printing but also inkjetcoating, where it had been difficult to make the alignment film 113thick because it has been necessary to make the viscosity of the varnishlow, become possible.

Embodiment 2

In embodiment 1, a film obtained by imidizing the polyamide acid is usedas the low-resistivity alignment film 1132 on the bottom of the 2-layeralignment film 113. The low-resistivity alignment film 1132 has norelationship with photoalignment, so a material can be selected focusingon making the resistance of the alignment film 113 small.

In liquid crystal display devices, a backlight is disposed on the backsurface of the liquid crystal display panel, and an image is formed bycontrolling the light from the backlight per pixel. A cold-cathode tubeor a light-emitting diode is used as the light source of the backlight.Consequently, the alignment film 113 becomes always exposed to the lightfrom the backlight while the liquid crystal display device is operating.When a photoconductive film is used as the low-resistivity alignmentfilm 1132 on the bottom, the electric charge with which the liquidcrystal layer has been charged can be let out more quickly, and DCafterimages can be alleviated more.

As a material by which the alignment film material exhibits aphotoconductive characteristic, for example, there can be listed NissanChemical's SE6414. In this case also, when this is mixed together withthe polyamide acid alkyl ester and the mixture material is applied ontothe substrate, the material phase-separates and becomes a 2-layerstructure. In this case also, the polyamide acid alkyl ester becomes thetop layer. Thereafter, heat is applied to imidize the material, but thisimidization can also be performed at the same time.

The above description has been given in regard to the alignment film 113of the TFT substrate 100, but the same is also true in regard to thealignment film 113 of the opposing substrate 200. The alignment film 113of the opposing substrate 200 is formed on the overcoat film 203, but inthis case also, the familiarity of the low-resistivity polymer 11, whichforms the low-resistivity alignment film 1132, with the overcoat film203 is strong, so the low-resistivity alignment film 1132 is formed onthe overcoat film 203 by leveling, and the photoalignment film 1131 isformed thereon.

It will be noted that effects can be obtained even when the alignmentfilm 113 with the 2-layer structure according to the invention isapplied just to the alignment film 113 of the TFT substrate 100 or theopposing substrate 200. This is because letting out the electric chargein the liquid crystal layer can raise a certain effect even from justeither substrate.

As described above, according to the invention, sufficient alignmentregulation resulting from photoalignment can be stably performed, andthe problem of a DC afterimage resulting from the photoalignment film1131 coming to have high resistivity can be solved.

It will be noted that the pretilt angle of the liquid crystal displaydevice to which the invention is applied becomes equal to or less than0.5 degrees according to measurement using the crystal rotation method.

The invention claimed is:
 1. A liquid crystal display device comprising:an alignment film formed by imidizing an alignment film varnishincluding a first compound and a second compound; and an electrodehaving a surface of Indium Tin Oxide; wherein the alignment film has afirst surface contacting a liquid crystal layer, and a second surfacecontacting the electrode, the first compound includes photodegradablepolymer, the second compound includes polyamide acid, the secondcompound has a higher affinity with an Indium Tin Oxide than the firstcompound, the alignment film includes a first polymer formed byimidizing the first compound and a second polymer formed by imidizingthe second compound, and the first polymer and the second polymer arephase-separated so as to dispose the first polymer on the first surfaceside and the second polymer on the second surface side.
 2. The liquidcrystal display device according to claim 1, further comprising a liquidcrystal layer, wherein, the liquid crystal layer is driven at IPS mode,and the alignment film is a photo alignment film.
 3. The liquid crystaldisplay device according to claim 1 further comprising: an organicpassivation film contacting with the alignment film; wherein the secondcompound has higher affinity with the organic passivation film than thatof the first compound.
 4. The liquid crystal display device according toclaim 1, wherein an imidization ratio of the first compound is largerthan that of the second compound.
 5. The liquid crystal display deviceaccording to claim 1, wherein a resistance of the first surface ishigher than that of the second surface.
 6. The liquid crystal displaydevice according to claim 1, wherein the second compound has aphotoconductive property.
 7. The liquid crystal display device accordingto claim 1, wherein the photodegradable polymer is a polyamide acidester.
 8. The liquid crystal display device according to claim 1,wherein a molecular weight of the first polymer formed by imidizing thefirst compound is larger than that of the second polymer formed byimidizing the second compound.